Active-matrix-driven organic electro luminescence (or organic light-emitting diode (OLED)) display devices and active-matrix-driven liquid-crystal display devices include thin-film semiconductor devices referred to as thin film transistors (TFTs).
In a display device of this type, TFTs are arranged in an array to form a TFT array device. Each pixel of the display device has a TFT for driving the pixel (driver transistor) and a TFT for selecting the pixel (switching transistor).
A pixel area where the pixels are disposed is surrounded by a peripheral circuit area where gate driver circuits and source driver circuits used for driving or switching of the pixels are disposed.
The driver transistor and switching transistor among transistors in a self-emitting OLED display device including OLED devices are required to have different characteristics. In order to increase drive performance of such OLED devices, the driver transistor is required to have excellent ON-state current characteristics, and the switching transistor is required to have excellent OFF-state current characteristics.
As for a display device including low temperature poly silicon (LTPS), such as a liquid-crystal display device, TFTs in peripheral circuit area and TFTs in the pixel area are required to have different characteristics. The TFTs in the peripheral circuit area are required to have excellent ON-state current characteristics, and the TFTs in the pixel area are required to have excellent OFF-state current characteristics.
A TFT is a transistor having a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode formed on a substrate. The channel layer is typically a silicon thin film. The silicon thin film comes in two main types. One is silicon thin film not crystalline (amorphous silicon film), and the other is a silicon thin film having crystallinity (crystalline silicon thin film).
In comparison with a TFT having a channel layer of an amorphous silicon thin film, a TFT having a channel layer of a crystalline silicon thin film has large carrier mobility and is thus excellent in ON-state current characteristics. Thus, use of a crystalline silicon thin film as a channel layer of a driver transistor is a known technique.
In a conventional method of forming a crystalline silicon thin film, an amorphous crystalline silicon thin film on a substrate is polycrystallized by applying heat to the amorphous crystalline silicon thin film with the addition of a metal catalyst. This method has the advantage that the amorphous crystalline silicon thin film can be crystallized at low temperature, while having a problem of higher cost due to a larger number of processes and a problem of difficulty in complete removal of metal elements after crystallization.
In another conventional method of forming a crystalline silicon thin film, a crystalline silicon thin film is formed on a substrate by chemical vapor deposition (CVD). This method has the advantage that manufacturing cost is lower because the number of processes is smaller, while having a problem of difficulty in achieving the ON-state current characteristics required for driver transistors because of microlite structure of the resulting amorphous crystalline silicon.
In another conventional method of forming a crystalline silicon thin film, an amorphous silicon film is polycrystallized by irradiating the amorphous silicon film with a light beam of an excimer laser having a predetermined energy density at a predetermined film formation temperature (Japanese Unexamined Patent Application Publication No. 07-235490 (Patent Literature 1)). However, this method has a problem of high running cost because of the laser system which shapes laser beams obtained by gas discharge and thus requires frequent overhaul.
There is another conventional method of crystallization of amorphous silicon using a laser, in which amorphous silicon film is changed into a polycrystalline silicon thin film by irradiating the amorphous silicon film with a pulsed laser having a wavelength of 532 nm (Japanese Unexamined Patent Application Publication No. 07-235490 (Patent Literature 2)).